The present invention relates to an electronic image sensing apparatus and, more particularly, to an electronic image sensing apparatus for sensing an optical image with an electronic image sensing element and processing the optical image as an electrical signal.
In general, in electronic image sensing apparatuses for receiving, as a still image, image sensing data serving as light intensity information from an object in photography, a very wide light intensity distribution from the object is detected by an image sensing element having a finite sensitivity width such as a silver halide film or CCD image sensing element. It is essentially important to determine an optimal exposure amount.
Of these apparatuses, a traditional image sensing system (optical camera) using a silver halide film has two opportunities of adjusting the luminance/image quality from the photography to final image printing on print paper in order to obtain the image sensing data as a "photograph with optimal brightness".
The first luminance/image quality adjustment is made when a photographer releases the shutter of the optical camera.
In this case, the photographer decides the shutter speed and F-number in accordance with his experiences, calculations based on the light amount measured by an exposure meter, or an automatic exposure control mechanism (AE mechanism) if incorporated in the optical camera. The shutter speed and F-number, which are supposed to be optimal, are determined to adjust the light amount.
The second luminance/image quality adjustment is made when an image is printed on print paper in a film development/printing shop (DPE shop).
It is checked whether the exposure in photography is overexposure or underexposure. If so, an exposure amount in printing is adjusted, and the brightness of the image data finally printed on the print paper is adjusted to be optimal.
FIGS. 9A and 9B explain a luminance correction process in printing an image on print paper.
In general, the luminance characteristic of print paper is almost linear at the central portion having an optimal luminance value in an original image recorded on a film.
The luminance characteristic of the print paper is curved at the two ends, low- and high-luminance regions of the image. The contrast difference cannot be clearly expressed in these regions.
In printing an image on print paper having the above characteristic, a printer automatically adjusts the luminance and determines an optimal luminance of an original image whose luminance distribution is optimal.
An overexposed image 91B and an underexposed image 91A have shifted peaks in the luminance distribution, as shown in FIG. 9A.
An image having a high luminance on a negative film is called an overexposed image on print paper, and an image having a low luminance on a negative film is called an underexposed image on print paper.
An operator in a DPE shop arranges an image in the optimal range of print paper, i.e., the optimal luminance range of print paper and obtains a photograph with optimal brightness in accordance with his experiences.
For example, the underexposed image 91A is adjusted toward a higher luminance so as to fall its luminance distribution within the optimal luminance range of the print paper. The underexposed image 91A is expressed as an image 92A (FIG. 9B) on the print paper.
Similarly, the overexposed image 91B is adjusted toward a lower luminance and is expressed as an image 92B (FIG. 9B) on the print paper.
In an electronic image sensing apparatus such as a conventional electronic camera apparatus for causing an image sensing element such as a CCD to photoelectrically convert a photographed image, process the resultant analog image sensing data, and output the image as image data, luminance adjustment of the image sensing data is made once as exposure control when the photographer releases the shutter.
FIG. 10 shows the processing operation from photography to data output in a conventional electronic image sensing apparatus, and FIG. 11 shows the arrangement of the conventional electronic image sensing apparatus.
In the conventional electronic image sensing apparatus, the photographer sets the shutter speed and F-number manually or using an automatic exposure control mechanism (AE mechanism) as in the optical camera using the silver halide film (step 100) and senses an image (step 101).
In response to the shutter release, an object image is detected by an image sensing unit 14 via a lens 11, a stop 12, and a shutter 13. The luminance of light from the object is temporarily stored as analog (voltage value) image sensing data in units of pixels.
The image sensing data is immediately read out and converted into a digital value of about 8 bits by a high-speed A/D converter 17 (step 102).
A digital signal processor 18 performs various types of digital signal processing. The processed data is stored in an image recording digital memory 19 as digital image data having, e.g., an 8-bit width.
The digital value having the 8-bit width is externally output from the digital signal processor 18, as needed.
In this conventional electronic image sensing apparatus, however, the image sensing data from the image sensing element is converted into normally an 8-bit digital value in the A/D conversion process (step 102).
When the luminance of the image data is to be adjusted later, the luminance resolution of the image further lowers. A fine change in luminance of the original image cannot be expressed even in a luminance resolution correction process. As a result, the image quality degrades.
In general, digital image data expressed by two-dimensional sample points have two important parameters, i.e., spatial resolution and luminance resolution.
A sample point constituting a digital image in the two-dimensional space is called a pixel. A value which expresses the smallest scale of a distance between two adjacent pixels is called the spatial resolution.
A value which expresses the smallest scale of the luminance value of each pixel is called luminance resolution. When any one of these values is unsatisfactory, the resultant image cannot be called a good image.
FIG. 12 shows an example in which a fine change in the luminance direction is lost.
Referring to FIG. 12, a dotted line 4 indicates the 1-bit digitalization result; a solid line 123, the 2-bit digitalization result.
When each sample point 122 of an analog image signal (original data) whose luminance finely changes is digitized in the 1-bit unit (two gradation levels) and 2-bit unit (four gradation levels), the fine change in luminance is lost with a decrease in bit width (digital resolution).
Once original data 121 from the image sensing unit is digitized at a small bit width, the luminance resolution greatly degrades. Any subsequent process cannot restore the lost luminance resolution, as can be apparent from FIG. 12.